Auxins and Gibberellins — Revision Notes

⚡ 30-Second Revision

Auxins: — IAA (natural), NAA, 2,4-D (synthetic).
Auxin Synthesis: — From Tryptophan.
Auxin Transport: — Polar (basipetal).
Auxin Effects: — Cell elongation, Apical dominance, Root initiation, Parthenocarpy, Prevent abscission, Xylem differentiation, Herbicide (2,4-D).
Gibberellins: — GA3 (most common).
Gibberellin Synthesis: — Mevalonic acid pathway.
Gibberellin Effects: — Stem elongation (dwarf plants), Seed germination ($\alpha$-amylase), Bolting, Fruit growth, Flowering.
Discovery: — Auxins (Darwin, Went), Gibberellins (Kurosawa, *Gibberella fujikuroi*).

2-Minute Revision

Auxins and Gibberellins are key plant growth promoters. Auxins, primarily IAA, are synthesized in shoot tips and young leaves. They exhibit unique polar transport and are crucial for cell elongation, leading to phototropism and gravitropism.

Other vital roles include establishing apical dominance, promoting adventitious root formation in cuttings, inducing parthenocarpy (seedless fruits), and preventing premature abscission. Synthetic auxins like 2,4-D are potent selective herbicides.

Gibberellins, with GA3 being the most studied, are synthesized in young leaves, seeds, and roots. Their most striking effect is dramatic stem elongation, especially in dwarf varieties, and inducing bolting in rosette plants.

They are essential for breaking seed dormancy by stimulating $\alpha$-amylase synthesis in the aleurone layer, and also promote fruit growth and flowering. Both hormones are vital for agricultural applications, enhancing crop yield and quality.

5-Minute Revision

Plant growth regulators (PGRs) like auxins and gibberellins are organic molecules that control plant development. Auxins, discovered through phototropism studies (Darwin, Went), are primarily Indole-3-acetic acid (IAA).

They are synthesized in apical meristems and young leaves, exhibiting unique polar transport. Their core function is cell elongation, which underlies phototropism and gravitropism. Auxins also enforce apical dominance (inhibiting lateral buds), promote adventitious root formation (e.

g., using NAA or IBA in cuttings), induce parthenocarpy (seedless fruits like tomatoes), and prevent premature abscission of young leaves and fruits. Synthetic auxins like 2,4-D are widely used as selective herbicides against broad-leaved weeds.

The 'acid growth hypothesis' explains auxin's mechanism: it lowers cell wall pH, activating enzymes that loosen the wall for expansion.

Gibberellins, discovered from the 'bakanae' disease of rice caused by *Gibberella fujikuroi* (Kurosawa), are a large family, with Gibberellic Acid (GA3) being prominent. They are synthesized in young leaves, seeds, and root tips.

Their most significant effect is dramatic stem elongation, particularly in genetically dwarf plants, by promoting both cell elongation and division in internodes. Gibberellins are crucial for breaking seed dormancy and promoting germination, notably by stimulating the aleurone layer to produce $\alpha$-amylase for starch breakdown.

They also induce bolting (rapid stem elongation before flowering) in rosette plants, enhance fruit growth (e.g., grapes), and can influence flowering. Both auxins and gibberellins are indispensable tools in modern agriculture for improving crop characteristics and yields.

Prelims Revision Notes

Auxins (Indole-3-acetic acid - IAA)

Discovery: — Charles Darwin & Francis Darwin (phototropism), F.W. Went (isolation from oat coleoptile tips).
Natural Auxins: — IAA, Indole-3-butyric acid (IBA).
Synthetic Auxins: — Naphthalene acetic acid (NAA), 2,4-Dichlorophenoxyacetic acid (2,4-D).
Synthesis: — Primarily in shoot apices, young leaves; precursor is Tryptophan.
Transport: — Polar transport (unidirectional, basipetal) through parenchyma cells.
Physiological Effects & Applications:

* Cell Elongation: Promotes growth in stems/coleoptiles (phototropism, gravitropism). Acid growth hypothesis. * Apical Dominance: Apical bud inhibits lateral bud growth. Removal of apical bud releases inhibition.

* Root Initiation: Promotes adventitious roots in stem cuttings (NAA, IBA). * Parthenocarpy: Induces seedless fruit development (e.g., tomatoes). * Abscission: Prevents premature fall of young leaves/fruits.

* Herbicides: 2,4-D as a selective herbicide for broad-leaved weeds in monocot crops. * Xylem Differentiation: Involved in vascular tissue formation.

Gibberellins (Gibberellic Acid - GA3)

Discovery: — E. Kurosawa (from *Gibberella fujikuroi* causing 'bakanae' disease in rice).
Types: — Over 100 GAs; GA3 is most common and studied.
Synthesis: — Primarily in young leaves, developing seeds, root tips; mevalonic acid pathway.
Transport: — Systemic via xylem and phloem.
Physiological Effects & Applications:

* Stem Elongation: Dramatic internode elongation, especially in dwarf varieties (e.g., dwarf pea, maize). Reverses dwarfism. * Seed Germination: Breaks seed dormancy. Stimulates $\alpha$-amylase synthesis in aleurone layer for starch breakdown.

* Bolting: Rapid internode elongation in rosette plants (e.g., cabbage, beet) before flowering. * Fruit Growth: Increases fruit size and length (e.g., grapes). * Flowering: Promotes flowering in long-day plants under short-day conditions; substitutes for vernalization.

* Malting Industry: GA3 speeds up malting process.

Key Differences to Remember:

Discovery: — Auxins (plant origin, phototropism), Gibberellins (fungal origin, disease).
Transport: — Auxins (polar), Gibberellins (systemic).
Dwarfism: — Auxins (no direct reversal), Gibberellins (reverse genetic dwarfism).
Seed Dormancy: — Auxins (no primary role), Gibberellins (break dormancy).
Herbicide: — Auxins (2,4-D), Gibberellins (none).

Vyyuha Quick Recall

All Apples Are Always Ready Particularly Herbicidal: Auxins, Apical dominance, Abscission prevention, Adventitious roots, Rooting, Parthenocarpy, Herbicides (2,4-D).

Giant Grapes Germinate Boldly: Gibberellins, Grape size, Germination (seed), Bolting.